There are posts on this forum that discuss converting pixel values to hectares, so try doing a Search for those. If you’re doing this conversion to hectares, you can simply add those values up within your catchment polygon to get total sequestration.
If you’re using ArcGIS, you can export the raster’s attribute table to a CSV table, which you can open in Excel. I’m not sure if QGIS has a similar function, but it might not since it doesn’t support raster attribute tables. If you find that QGIS does have a raster table export function, please let us know!